The present invention relates to the general subject of circuits for powering discharge lamps. More particularly, the present invention relates to a ballast that includes a circuit for detecting and responding to electrical disturbances such as output arcing.
Ballasts for gas discharge lamps provide high ignition voltages for starting the lamps. The ignition voltages supplied by preheat type ballasts are typically on the order of several hundred volts (e.g., 500 volts peak), while those provided by instant-start type ballasts may exceed 1000 volts peak. As a consequence of these high output voltages, ballasts are subject to the problem of output arcing.
Output arcing may occur in any of a number of different ways. For example, in fluorescent lighting installations, it is a common practice to replace failed lamps while AC power is applied to the ballast. This practice is referred to as xe2x80x9clivexe2x80x9d relamping. During live relamping, as a lamp is being removed or inserted, a momentary arc may form between the fixture socket contacts and a pin of the lamp. As another example, a sustained arc (as opposed to a momentary arc) arc may occur due to poor connections in the output wiring or the lamp sockets, or if a lamp is improperly installed such that a small gap exists between the lamp pins and the contacts within the fixture sockets.
Arcing is generally acknowledged to cause degradation of the contacts in the fixture sockets and undue stress on components within the ballast. Sustained arcing is especially undesirable. In order to minimize any ill effects due to arcing, it is important that the arc be promptly extinguished. Thus, a need exists for a ballast having a protection circuit that quickly detects an output arc and then takes appropriate action to quickly extinguish the arc. A further need exists for a protection circuit that is economical and easy to implement within existing ballasts. Such a ballast and protection circuit would represent a considerable advance over the prior art.